Earth-boring tools for forming wellbores in subterranean earth formations generally include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements fixedly attached to a bit body of the fixed-cutter drill bit. Similarly, roller cone earth-boring rotary drill bits include cones that are mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which it is mounted. A plurality of cutting elements may be mounted to each cone of such roller cone drill bit.
The cutting elements used in fixed-cutter, roller cone, and other earth-boring tools often include polycrystalline compact cutting elements, e.g., polycrystalline diamond compact (“PDC”) cutting elements. The polycrystalline compact cutting elements include cutting faces of a polycrystalline compact of diamond or another hard material (collectively referred to herein as “hard material”). Grains or crystals of the hard material are inter-bonded within the polycrystalline compact. (The terms “grain” and “crystal” are used synonymously and interchangeably herein.) Thus, the polycrystalline compacts include direct, inter-granular bonds between the grains or crystals of hard material.
Polycrystalline compact cutting elements may be formed by sintering and bonding together relatively small grains of the hard material in the presence of a metal solvent catalyst under high temperature and high pressure conditions (referred to herein as “high-pressure, high-temperature processes” (“HPHT processes”) or “high-temperature, high-pressure processes” (“HTHP processes”)). The HPHT process forms a layer or “table” of polycrystalline diamond material (or alternative hard material), which may be formed on or later joined with a cutting element substrate. The cutting element substrate may comprise a cermet material (i.e., a ceramic-metal composite material) such as, for example, cobalt-cemented tungsten carbide.
The metal solvent catalyst may include, for example, cobalt, iron, nickel, or alloys and mixtures thereof. The catalyst, which may initially be in a powdered form, may be mixed with the grains of hard material prior to sintering the grains together in the HPHT process. Alternatively or additionally, when a polycrystalline compact is formed on a cutting element substrate that includes a material such as cobalt, the cobalt, or other such material, from the substrate may be swept into the grains of hard material during the sintering process and may serve as the catalyst material for forming the inter-granular bonds between the grains of hard material. For example, cobalt from a substrate may be swept into overlying diamond grains of a diamond table to be formed and may catalyze the formation of diamond-to-diamond bonds.
Upon formation of a polycrystalline table using an HPHT process, catalyst material may remain in interstitial spaces between the grains of hard material in the resulting polycrystalline table. The presence of the catalyst material in the polycrystalline table may contribute to thermal damage in the polycrystalline table when the cutting element is heated, during use, due to friction at the contact point between the cutting element and the earth formation. To reduce the amount of catalyst material remaining in the polycrystalline table after formation, and, therefore, to reduce the likelihood of subsequent thermal damage during drilling, smaller grains of hard material may be included in the interstitial spaces between grains of hard material. Where the hard material is diamond, the smaller, fine grains may also be diamond. These interstitial fine grains of hard material increase the amount of hard material included in the polycrystalline table formed, lower the amount of catalyst needed to fill remaining interstitial space, and contribute to the hardness and strength of the cutting element while lessening the risk of thermal damage.
The HPHT process may be carried out by introducing the grains of hard material and, optionally, the catalyst material intermixed with the grains to a press (e.g., a diamond press, a cubic press, or other such press) either with or without a substrate. The press is configured to subject the materials therein to extreme pressures and temperatures. The pressure and power input can be adjusted, with adjustments to the power input yielding a change in the temperature within the press. Due to the extreme temperatures involved, the temperature within the HPHT system may not be directly monitored, but may be a factor of the power input.
A conventional HPHT process for forming a diamond table of a PDC may generally include a raise in pressure until the diamond material is near a sintering range. Power may then be steadily applied to provide essentially a steady temperature. The steady application of power at steady application of pressure provides for a relatively flat interval, or “plateau,” which may be held for several minutes or longer at a relatively steady pressure and temperature. Thereafter, the temperature and pressure of the system are dropped to well below the diamond stable region before the formed parts are removed from the HPHT system. This HPHT process may be represented by temperature versus time and pressure versus time profiles such as those illustrated in FIGS. 6 and 7.
In a conventional HPHT process, during the plateaus, i.e., the steady applications of pressure and temperature, the hard material is sintered, and inter-granular bonds are formed between the grains of hard material. The conventional HPHT process is, however, subject to a trade-off between encouraging formation of desirable inter-granular bonds and discouraging undesirable grain growth, chemical breakdown of the hard material, and other impairments to the physical properties of the table to be formed. More particularly, high temperatures and pressures encourage formation of inter-granular bonds. However, at temperatures at or above about 750° C., some of the hard material crystals within the hard material table may react with catalyst material, causing the hard material to undergo a chemical breakdown or conversion. For example, diamond may breakdown or be converted to another allotrope of carbon, or diamond crystals may graphitize at the diamond crystal boundaries, which may substantially weaken the diamond table. Also, at extremely high temperatures, in addition to graphite, some of the diamond crystals may be converted to carbon monoxide and carbon dioxide. Also, at high temperatures and/or pressures, nanoparticles of the hard material included within the structure may dissolve into catalyst material and later redeposit on solids within the structure, thereby contributing to unwanted grain growth. In other words, the grains of hard material may dissolve at elevated temperatures and pressures and later redeposit on other grains, causing changes to the microstructure.
The physical properties of the body of the polycrystalline compact are dependent on the microstructure. Dissolving and redepositing hard material on other grains may result in formation of larger-grain microstructures, which presents the properties of such larger-grain microstructures. The properties of the larger-grain microstructures may not be desirable and may negatively impact the table's thermal stability, wear resistance, and toughness during subsequent use.
Nonetheless, when forming, by conventional HPHT processes, cutting elements including small, fine grains of hard material within interstitial spaces between larger, coarse grains of hard material, the maximized temperature and pressure conditions that accommodate formation of wanted inter-granular bonds may conflict with efforts to avoid dissolution and redeposition of fine grain material and resulting grain growth. This trade-off in conventional HPHT conditions may produce less inter-granular bonding than desired and more material dissolution and redeposition than desired.